The present invention relates to ultrasonic imaging and in particular to an apparatus and method for making ultrasonic elastography measurements.
Elastography is an imaging modality that reveals the stiffness properties of tissue, for example, axial strain, lateral strain, Poisson's ratio, Young's modulus and other common strain and strain-related measurements. In elastography, strain measurements may be collected over an area and compiled as a two-dimensional array of data which may be mapped to a grey or color scale to form a strain “image.” Analogously, strain measurements may be collected over a volume displayed either three-dimensionally or as a series of stacked two-dimensional images.
In quasi-static elastography, two images of tissue (“pre-compression” and “post-compression”) are obtained by the ultrasound device with the tissue in two different states of compression, for example, no compression and a given positive or negative (tensile) compression. The tissue may be compressed by an external agency such as a probe or the like, or muscular action or movement of organs near the tissue. Strain may be deduced from these two images by computing gradients of the relative local shifts or displacement in the images along the compression axis. Quasi-static elastography is analogous to a physician's palpation of tissue in which the physician identifies firm structures by pressing the tissue and detecting the amount the tissue yields under this pressure.
Determining the relative displacement of the tissue between the two compression images is normally done by analyzing successive portions of the ultrasonic signal in a series of discrete 1-D windows or 2-D or 3-D kernels. The windows define portions of the ultrasonic signal at successive times representing reflections from tissue at successive locations along the path of the ultrasound. Kernels denote 2-D or 3-D search regions in the 2-D or 3-D received ultrasound echo signals in the B-mode or RF data. The ultrasonic signal may be either an envelope of the amplitude of a received ultrasonic echo or the echo signal (RF) itself.
Generally, the signal in each window in the pre-compression image is cross-correlated to the signal in a search area of the post-compression image to find corresponding window in the post-compression data and thereby determine slight shifts between the signals and thus shifts in location of the underlying tissue with compression. This cross-correlation process is repeated for successive windows of the ultrasonic signal yielding local displacement of tissue for each window. The gradient of these local displacements yields a measure of the local strains in the tissue.
The resolution of elastography is fundamentally limited by the size of the windows used to determine the displacement of the tissue. Currently, larger windows are used, for example, on the order of twenty wavelengths (a centimeter or more at common ultrasound frequencies), too large to effectively image extremely smaller objects such as the calcifications that accompany breast cancer.
Smaller window sizes, for example, on the order of one wavelength (less than a millimeter at common ultrasound frequencies), potentially provide an increase in the resolution of elastography, but are practically limited by problems of tissue displacement moving the echo signals that arise from this tissue to be entirely out of the window and increased statistical miscorrelation as the amount of correlated data is reduced and the discipline of only correlating within corresponding windows is relaxed.
The problem of post-compressed tissue failing to remain within corresponding windows of the post-compressed tissue can be addressed by using larger size windows for the post-compression data or by temporal stretching of the post-compression data to improve the alignment of the tissue between windows. This latter approach tends to introduce artifacts into the post-compression data where some regions are over stretched while other regions are under stretched. The former approach still faces the problem of statistical mismatching in the cross correlation process promoted by the unmatched window sizes.